Internal combustion engines, boilers, power generation equipment, and equipment that burn a fuel typically produce exhaust or stack emissions that can include byproducts such as nitrogen oxides and sulfur oxides. Government regulations may require that sources of these byproducts include a device to monitor the quantity or concentration of particular gases that are emitted during operation. Real-time measurement of these byproducts can also be used by closed-loop control systems to regulate the operation of an engine or other combustion system. Monitoring devices can generally be classified into one of three fundamental types based on how the exhaust or emission is sampled, including in-situ, extractive, and dilution extractive.
The use of the same reference symbols in different drawings indicates similar or identical items.